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There is the theoretical answer and the practical real life answer. In theory one is more precise than the other. By the time you figure in the width of the marking line and inaccuracy of hand held scissors you have left the theoretical world in the far off distance.

I may be slow... But I sure am gimpy.

"Bless you child, when you set out to thread a needle don't hold the thread still and fetch the needle up to it; hold the needle still and poke the thread at it; that's the way a woman most always does, but a man always does t'other way."
Mrs. Loftus to Huck Finn

I made a template out of cardboard, by hanging a string the length of the curve, I marked the bottom of the curve by, I think, .5" for every 1 foot of length. Then I hung the cardboard upright, and traced the curve of the string onto the cardboard. After I made the template, I used a roller cutter following the curve to make an exact cut. That being said, it all depends on how well and precise the material is hemmed too.

Dwight

Psalm 19:1-3 "The heavens declare the glory of God; and the firmament sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. There is no speech nor language, where their voice is not heard."

There is the theoretical answer and the practical real life answer. In theory one is more precise than the other. By the time you figure in the width of the marking line and inaccuracy of hand held scissors you have left the theoretical world in the far off distance.

That's what I say. I calulated it using a MATLAB program someone else wrote. I think sewing and cutting make a bigger difference. Mine had a couple small winkles sewn in. More from my error than anything else.

If you do some searching there are countless threads dicussing Math vs a curved stick for cat cuts.

Is that too much to ask? Girls with frikkin' lasers on their heads?
The hanger formly known as "hammock engineer".

I have made a few ( more than 10 ) cat cut tarps. I use the string through the tube method to create the arch. The one thing that I have learned is to make the arch less extreme than I want the final product.
yis,
Erin

I've got a question about cat cut edges on a tarp. When using the spreadsheet, does the calculation take into account the material needed for the rolled hem? I havn't looked at the file myself yet, but it would seem to me that your cuts could end up as much as an inch off if you don't allow for it. Is the true?

Dave,
Don't know the answer about the hem, but the formula is for catenary curve only (cable curve) and does not account for the effect of pulling the fabric on the bias. That factor blows all the calculations. The eyeball is the best judge. Bill Moss of Moss Tents was the great master of compensating for bias. He just eyeballed it and fiddled until he got it right. Then he ripped out the seams and used the prototype to make patterns. Different fabrics with different coatings vary in their distortion through the bias, and no algorithm can compensate for all that.

The first time I made a hex tarp with curves on the hems alone, it took me two tries to get enough offset in the curves. It was lots more than I had anticipated. The hems along the bias took a full 1.25 inch per foot offset whereas the hems aligned square to the weave took only 3/8 inch per foot. Offset refers to the distance at the center of a hem from the straight edge to the curved edge. Please note the caution above: it depends on the fabric and the angle of the hem to the weave. What I used may not work on your project.

When you fold (and sew) a curved edge, the folded edge does not line up precisely. Only with straight edges does that happen. You have to make the fabric do something it does not naturally want to do... and silynylon is slippery and cantankerous without asking it to do something unnatural. Experience is a big help.

If you look in some of the sewing books you will see they cut darts around neck lines to relieve the buckling(?) that naturally wants to occur. I wouldn't do that on these curved edges on tarps but you have to deal with it a little differently than when you hem straight edges.